1. Field of the Invention
The invention relates to a gear-type pump. Gear-type pumps of this type comprise an externally toothed inner rotor which rotates within an internally toothed outer rotor which rotates eccentrically thereto and is in turn mounted in an eccentric ring.
2. Description of the Prior Art
To achieve that the gear-type pump with alternating drive direction maintains its pumping direction the eccentric ring which governs the position of the outer rotor must be pivoted through 180.degree.. The simplest way of doing this is to retain always a certain frictional engagement between the outer surface of the outer rotor and the inner surface of the eccentric ring, the eccentric ring on rotation of the outer rotor thereby always being pressed against an end stop corresponding to the direction of rotation. To avoid the constant friction losses of such a pump it has been proposed in German Auslegeschrift No. 2 055 883 instead of the direction frictional contact between outer rotor and eccentric ring to mount these two parts in each other with a certain play and to provide in the eccentric ring a radially movable piston which is pressed by a spring against the outer rotor and carries a friction body on its surface facing said rotor. The frictional engagement is thus no longer as hitherto directly between the eccentric ring and outer rotor but established indirectly between these components through the intermediary of the friction body.
The aforementioned piston defines an expansion chamber which is connected via a pressure line to the pressure connection of the pump.
In operation of the pump the fluid pressure transmitted from the pressure connection to the expansion chamber acts on the piston, counteracts the spring loading said piston and lifts the friction body out of frictional engagement with the outer rotor. Now, if on reversal of the direction of rotation the pump briefly comes to a stop the delivery pressure drops and thus also the pressure in the expansion chamber, the friction body establishes frictional engagement between the eccentric ring and outer rotor and on starting up in the opposite direction the outer rotor therefore entrains the eccentric ring into its second end position in which the eccentric ring is held by a stop. The delivery pressure building up then again acts in the expansion chamber on the piston so that during operation of the pump the frictional engagement is cancelled.
In spite of its advantages the known pump has the disadvantage that the arrangement of cylinder-like expansion chamber, piston, spring and friction body is relatively complicated and difficult to assemble. In addition, the friction body because of its small dimensions is subjected to high wear. Furthermore, problems are encountered when for example in trucks viscous oil is to be pumped. Moreover, the piston-cylinder arrangement has a considerable radial height and this increases the outer diameter of the eccentric ring and thus also of the pump. Finally, the eccentric ring is limited in its minimum thickness by the piston diameter and the necessary wall thickness.